Fix crash on presenting pan modal over pan modal

[ppamorim]

https://github.com/slackhq/PanModal/pull/176/files

func haltScrolling(_ scrollView: UIScrollView) {
         let point = CGPoint(x: 0, y: min(scrollViewYOffset.rounded(), scrollView.contentSize.height))
         if presentedViewController.isPanModalPresented && point != scrollView.contentOffset {
             scrollView.setContentOffset(point, animated: false)
             scrollView.showsVerticalScrollIndicator = false
         }
     }

@objc
     open var isPanModalPresented: Bool {
         return (transitioningDelegate as? PanModalPresentationDelegate) != nil
     }